The present invention relates to a method for controlling a drivetrain of a motor vehicle, which comprises a drive engine and a dual-clutch transmission, the dual-clutch transmission comprising a first friction clutch and a second friction clutch together with a first transmission unit and a second transmission unit, in order to establish a first power transmission branch and a second power transmission branch, the dual-clutch transmission comprising a clutch actuator device for actuating the friction clutches and a gearshift actuator device for engaging and disengaging gears of the dual-clutch transmission.
Drivetrains having such dual-clutch transmissions are generally known. Here, through overlapping actuation of the two friction clutches, drive power can be transferred from the one power transmission branch to the other power transmission branch without any interruption in tractive force. The one transmission unit generally contains the odd gears and the other transmission unit the even gears. The reverse gear is also preferably integrated into the transmission unit with the even gears.
The clutch actuator device may comprise electric motors as actuators. It is preferable, however, if the friction clutches are each actuated by means of suitable hydraulic actuators.
The gearshift actuator device may likewise comprise hydraulic actuators. The provision of a gearshift hydraulic cylinder as actuator for each shift clutch assembly (which comprises two adjacent shift clutches in each case) is known in this case. Also known is the use of electric motors for actuating the shift clutches of the transmission units. Here the gearshift electric motors may be arranged concentrically with the shift clutch.
In a further known embodiment the gears of the dual-clutch transmission are actuated by means of at least one shift drum. Here the shift drum is preferably driven by means of an electric motor, but it may also be driven by a hydraulic drive.
In a variant the dual-clutch transmission comprises just one shift drum. In this embodiment the gear changes are as a rule purely sequential. Multiple gear shifts through one or more gears are in this case relatively time-consuming.
In a further variant the provision of a separate shift drum for each transmission unit is known, the shift drums being capable of actuation independently of one another. In this embodiment, too, a problem remains with regard to the relatively long shifting times when shifting through multiple gears.
The present control method assumes a state in which the vehicle is operated with a high original gear. If multiple downshifting from this initial state is required, for example in the event of a kickdown or a braking process, with dual-clutch transmissions, the gearshift actuator device of which comprises one or two shift drums, relatively long delays may occur until the correct gear is engaged.
In the event of a kickdown, in which a specific travel of the accelerator pedal is generally reached or exceeded, a lowest possible target gear is calculated on the basis of the prevailing driving condition. The downshift from the high original gear into the lowest possible target gear is then performed, which in some cases can take a relatively long time, however, and is not desirable especially when embarking upon overtaking maneuvers or the like.